FY 2019

Quantifying the role of oceanic feedbacks on ENSO asymmetry

Guan, C., M.J. McPhaden, F. Wang, and S. Hu

Geophys. Res. Lett., 46(4), 2140–2148, doi: 10.1029/2018GL081332, View online (open access) (2019)

Abstract. The role of oceanic feedbacks in determining the asymmetry of El Niño–Southern Oscillation (ENSO) magnitude, spatial structure, and duration is quantified on the basis of a novel temperature variance budget. Results confirm previous studies that in the eastern Pacific, El Niño warm temperature anomalies are larger in magnitude than La Niña cold temperature anomalies mainly due to stronger positive oceanic feedbacks for El Niño. We find that La Niña cold anomalies are typically stronger than El Niño warm anomalies in the central Pacific with a faster growth rate for cold anomalies, due to a stronger positive thermocline feedback and weaker nonlinear damping. The thermocline feedback related to recharge oscillator dynamics plays a dominate role and leads to asymmetry in the duration of El Niño and La Niña events. In particular, the thermocline feedback becomes significantly negative during the late decaying phase of El Niño and speeds up its demise.

Plain Language Summary. The El Niño–Southern Oscillation (ENSO) is well known to have profound impacts on global climate. Many asymmetric features exist between its warm phase of ENSO (El Niño) and cold phase (La Niña), but their causes are still not fully understood. Our study examines three aspects of ENSO asymmetry: (1) the amplitude of anomalous temperature during the mature El Niño events is larger than La Niña in the equatorial eastern Pacific; (2) in the equatorial central Pacific, the amplitude in mature phase of La Niña is larger than El Niño; and 3) La Niña typically lasts longer than El Niño events. We find that the larger amplitude of El Niño than La Niña is due to stronger positive feedback for El Niño in the eastern Pacific. In the central Pacific, La Niña has a faster growth rate than El Niño, which may be induced by stronger positive thermocline feedback in the developing phase and less nonlinear damping effect. The asymmetry of ENSO duration is because the decay rate for La Niña events is slower than for El Niño, as a result of positive thermocline feedback that change sign to negative for El Niño but not for La Niña.